Electron impact spectrometer with an improved source of monochromatic electrons

ABSTRACT

An electron source for generating a high intensity electron beam, the electrons of which have a limited number of discrete kinetic energies is provided. The electron source which could be used e.g. in an electron impact spectrometer, comprises a monoor di-chromatic light source, e.g., a neon plasma the light of which injected into a gas, preferably a noble gas, in order to generate photo electrons having a limited number of discrete kinetic energies.

United States Patent 11 1 Lindholm et al.

1 ELECTRON IMPACT SPECTROMETER WITH AN IMPROVED SOURCE OF MONOCHROMATICELECTRONS [76] Inventors: Carl Einar Lindholm, Sigynvagen 5,

182 64 Djursholm, Leif Gosta A 7 Asbrink, Arbetargatan 24A, 11245Stockholm, both of Sweden [22] Filed: Mar. 23, 1973 [21] Appl.'No.:344,116

Related US. Application Data [63] Continuation of Ser. No. 137,162,April 26, 1971,

abandoned.

[30] Foreign Application Priority Data May 27, 1970 Sweden 7286/70 [52]US. Cl 250/305, 250/306, 250/307 [51] Int. Cl G0ln 23/00, G01t 1/36 [58]Field of Search 250/281, 305, 306, 307,

[56] References Cited UNlTED STATES PATENTS 2,457,530 12/1948 Coggeshallet a1 250/281- 1451 Apr. 23, 1974 Omura ct al 250/423 X Hillier 250/305OTHER PUBLICATIONS High Resolution, Low Energy Electron Spectrometer, byJ. A. Simpson from The Review of Scientific.

Instruments, Vol. 35, No. 12, Dec., 1964, pages Primary Examiner-WilliamF. Lindquist 57] ABSTRACT An electron source for. generating a highintensity electron beam, the electrons of which have a limited number ofdiscrete kinetic energies is provided. The electron source which couldbe used e.g. in an electron impact spectrometer, comprises a monoordichromatic light source, e.g., a neon plasma the light of whichinjected into a gas, preferably a noble gas, in order to generate photoelectrons having a limited number of discrete kinetic energies.

4 Claims, 3 Drawing Figures FECORDE z +20V 132 I i M/L T/PL/EF ELECTRONIMPACT SPECTROMETER WITH AN IMPROVED SOURCE OF MONOCHROMATIC ELECTRONSThis application is a continuation of Ser. No. 137,162, filed Apr. 26,1971, now abandoned.

Apparatus for irradiating a sample with electrons having a limitednumber of discrete kinetic energies.

The present invention refers to an apparatus for irradiating a samplewith electrons having a limited number of discrete kinetic energies,e.g., in an electron impact spectrometer.

When analyzing and identifying molecules or atoms in a sample theenergies of the different electron orbits in the molecules of the samplecan be investigated. It is, e.g., possible to study the ionization ofatoms and molecules by means of photoelectron spectrometry. The sampleis then irradiated by X-ray quanta or ultraviolet light. Due to thisirradiation photoelectrons having a certain kinetic energy are detachedfrom the atoms. The kinetic energy of the photoelectron will then bedetermined by a difference between the energy of the radiation and thebinding energy. The kinetic energy of the photoelectron is determined byfeeding the electrons to an analyzer, i.e., a device through which onlyelectrons having a certain predetermined energy are able to pass. Theanalyzer may consist of two concentric spherical or cylindricalelectrodes between which the photoelectrons pass. If the voltage acrossthe electrodes is chosen in a suitable way and apertures are arranged atthe input and output of the analyzer only electrons having apredetermined kinetic energy will pass through the analyzer. By varyingthe potentials of the electrodes this energy can be changed and thenumber of electrons as a function of their kinetic energies can bedetermined. This photoelectron spectrometer is described in details e.g.in Analytical Chemistry, 42 No. 1 Jan. 1970 pp. 2OA-4OA.

Supplementary investigations of the structure of atoms and molecules canbe performed by means of excitation. The excitation can be observedoptically by studying the absorption in the sample of ultraviolet lightof different wavelengths or by using electron impact spectrometry.Optical investigations are advanta geous in that the energy resolutioneasily could be made very high whereas they suffer from the drawbackthat the intensity is difficult to define and investigations of higherexcitation energies than 7 eV requires expensive and complicated vacuumspectrographs. Electronic investigations by using electron impactspectrometry do not suffer from these drawbacks.

The intensity determination is very reliable and the complete energyrange can be investigated in one sweep without any limitations. Inchemical and molecule physical studies the electron impact spectrometeris thus a very useful tool. The only important drawback ofthisinstrument has up until now been that the energy resolution is not asgood as in the optical spectrometers.

In an electron impact spectrometer the sample is exited by electronshaving a certain kinetic energy. When the electrons meet the sample theylose part of their kinetic energy and excite the atom. By determiningthe loss of energy of the electrons the exitation energies of the samplemay be determined. In order to obtain mono-energetic electrons one hashitherto used a conventional electron gun, consisting of a glow cathodeand an acceleration electrode and thus generated electrons havingkinetic energies within a certain range. This range will then be atleast some 250 meV. In order to obtain mono-energetic electrons amono-chromator must thus be used. This device could, e.g., consist oftwo concentric cylindrical or spherical electrodes and it could bedesigned as the above described electron energy analyzer. Since amono-chromator having a high resolution will have a very low degree oftransmission, the number of electrons obtained from the'monochromatorwill decrease rapidly with a decreasing bandwidth of the mono-chromator.

From the description above it is obvious that it is desirable to providea device in which the sample is excited by electrons having a highintensity as well as a high degree of energy uniformity. The purpose ofthe present invention is then to provide such a device. Thecharacteristics of the invention will appear from the enclosed claims.

The'invention will now be described in detail in connection with theenclosed drawing in which:

FIG. 1 shows an electron impact spectrometer known per se;

FIG. 2 shows an electron source used in the arrangement according to theinvention; and

FIG. 3 shows an electron impact spectrometer in which the arrangementaccording to the invention is utilized.

FIG. 1 shows an -electron impact spectrometer d esc ribed e. g. byLassettre (J. Chem. Phys, 48, 5066 (1968)). The spectrometer consists ofan electron source comprising a glow cathode l and an acceleratingelectrode 2. In the electron source electrons emit-' ted from the glowcathode are accelerated by the electrode 2 to an energy which issuitable for analysis in a mono-chromator. Because of the fact that theinitial energy of the electrons emitted from the glow cathode are spreadwithin a fairly wide range a corresponding spread of the acceleratedelectrons will be obtained. In

order to obtain the mono-energetic electrons the electrons from the gunare therefore supplied to a monochromator consisting of two cylindricalor spherical electrodes 3, 4. The electrode 3 is then negative inrelation to the electrode 2 and the electrode 4 is positive in relationto the same electrode. Electrons injected into the mono-chromator willthus be deflected in the space between the electrodes. Two apertures 5,6 are arranged at the input and output of the monochromator respectivelywhich implies that only electrons which are located within a certainenergy range will pass through the mono-chromator. The monochromatorcould then be designed so as to make this energy range as small asrequired. However, if the energy range in the mono-chromator isdecreased, the transmission will likewise decrease, i.e., the number ofelectrons passing through the mono-chromator will be very small. Theelectrons obtained from the monochromator are accelerated to a suitablevelocity by means of an electrode 7 and they are supplied to an impactchamber 8 in which the sample to be investigated is introduced. Theimpacting electrons will then lose some of their energy when they meetthe gas molecules in the chamber and thus leave the chamber with areduced energy. The electrons leaving the chamber will then pass throughan aperture in an electrode 9, the potential of which determines thevelocity .of the electrons supplied to an analyzer of the same design asthe monochromator and consisting of two electrodes 10, 11. By varyingthe potentials of the electrodes of the analyzer electrons havingdifferent kinetic energies will pass between the electrodes. The outputof the analyzer is connected to a detector 12, preferably an electronmultiplier, provided with an aperture 13, the output of the multiplierbeing connected to a recorder 14, e. g. a plotter. In this plotter onecould thus obtain a curve that defines the number of electrons as afunction of the energy loss and in-this way the sample in the chamber 8could be analyzed as to quantity as well as to quality. The drawback ofthe above described device is that the resolution of the instrument islimited by the energy spread of the electrons that are injected into thesample. It is thus very essential to provide an electron source thatirradiates electrons within a very narrow energy range.

In FIG. 2 an electron source used in the apparatus according to theinvention is shown, this electron source making it possible to obtainelectrons within a much narrower energy spectrum than previouslypossible. In FIG. 2 reference 15 denotes a container containing, e.g.,neon. The neon is supplied to a microwave cavity 17 via a pressurereducing valve 16, the microwave cavity being connected to a microwavegenerator 18, e.g'., a magnetron. The magnetron generateselectromagnetic waves of a frequency of, eg. 2.5 GHz. The neon will thusthen be exited and generate'photones having the energies 17.85 and 16.67eV. The energy uniformity of the generated radiation will then beextremely high. The microwave cavity 17 is connected to a vacuum tank 20via a tube 19, the tank in turn being connected to a brass cylinder 22via a tube 21. The neon will then be evacuated from the vacuum tank 20whereas the radiation passes into the brass cylinder. The other end ofthe brass cylinder is connected to a gas container 24, containing, e.g.,argon, via a reducing valve 23. The neon light supplied to the cylinderwill then generate photoelectrons from the argon, these photoelectronshaving extremely well defined energy levels. Thus photoelectrons havingthe energies 0.73 eV, 0.91 eV, and 1.09 eV are obtained, the spreadingwithin the respective energy levels being about 2 meV, i.e., a very highenergy uniformity will be obtained. These electrons leave the brasscylinder via an aperture 25 surrounded by a brass plate 26. ()utside thebrass plate 26 another brass plate 27 is arranged, a retarding voltagebeing supplied to this latter plate. The voltage obtained from a voltagesource 28, having its positive terminal connected to the plate 26, isthen preferably somewhat higher than 1 eV so that only electrons havingthe energy 1.9 eV will pass the plate 27, the energy of these electrons,thus being reduced to about 0.09 eV. By using the device defined aboveextremely mono-energetic electrons could thus be obtained. One coulde.g. easily obtain an intensity sufficient for a spectrometer whilekeeping the electron energies within a range which is less than meV. Inorder to obtain the above mentioned energy uniformity is, however, veryessential that no disturbing fields are present in the container 22. Itis thus very essential that no surface charges are present on the insideof the container. During experiments it has been found that thesesurface charges could be avoided if the surface is pro vided with a thinlayer of colloidal graphite. It is also obvious that the mono-chromatorcould be designed in accordance with the mono-chromator'shown in FIG. 1

but having a much lower resolution (0.1 eV) and consequently a highertransmission. A magnetic monochromator could also possibly be used butsuch a mono-chromator would probably give rise to disturbing fields inthe brass cylinder.

In FIG. 3 an electron impact spectrometer using the electron source ofFIG. 2 is shown. The spectrometer comprises the brass container 22, theaperture 25, the brass plate 26 and the retarding electrode 27. Theelectrode 27 is then connected to earth whereas the container and theaperture 26 have the potential 1 V. The spectrometer further comprisesan impact chamber 8 in which the sample to be analyzed is introduced.

The impact chamber has a potential of +300 V. Consequently the electronsfrom the .electron source will have a high velocity within the chamberwhere they meet the molecules of the specimen. Some electrons will thenlose an amount of energy corresponding to the excitation energies of thesample. After the chamber a brass plate 29 with an converging aperture30 is arranged, the plate having a potential of +20V. The kinetic energyof the electrons leaving the aperturej30. is thus 20 eV minus the lossof energy deriving from the excitation in the chamber. It is thusessential that the loss of energy of the electrons passing the chamberis less than 20 eV as otherwise the electrons will not pass the plate29. However, most substances have excitation energies below 15 eV andthus 20 eV is a suitable voltage'as it will give a veryhigh resolutionof the different energy levels of the specimen. After passing theaperture 30 the electrons are transferred into an analyzer consisting oftwo spherical or cylindrical electrodes 10, 11, the analyzer havingessentially the same design as the analyzer of FIG. 1. Electrons leavingthe analyzer pass an aperture 13 having the same potential as theaperture 30, i.e., no further acceleration is obtained in the analyzer,and pass into an indicating apparatus 12, suitably an electronmultiplier. The output of the indicator is connected to a recorder 14,e.g., a plotter. The potentials of the electrodes 10 and 11 arepreferably symmetrical to the potential of the aperture 30 andfurthermore these potentials can be varied in order to obtain thespectrum in the plotter 14 of electrons having different energies. It isof course also possible to obtain a spectrum by varying the potential ofthe plate 29. Thepotential drop between the plate 29 and the electrodesl0, l1 and 13 should than be constant.

In connection with this it should also be emphasized that it is notnecessary to make the electrons generated in the brass cylinder pass amono-chromator before they are supplied to the chamber 8. It is namelyalso possible to evaluate the energy losses of the electrons even ifthey originally have more than one discrete kinetic energy. It shouldalso be pointed out that the above described device could be used as aphoto electron spectrometer by removing the chamber 8 and introducingthe specimen in thebrass cylinder.

According to the invention an electron source from which low energyelectrons having a high degree of energy uniformity is provided. The useof such an electron source is of course not limited to an electronspectrometer. Mono-chromatic electrons .could, e.g., be used in massspectrometers where ionized molecules or parts of molecules formed inelectron impacts are in vestigated. Mono-chromatic electrons could alsobe used in diffraction investigations of crystals.

We claim:

1. Apparatus for bombarding a sample with electrons having a limitednumber of discrete kinetic energies, characterized in that the apparatuscomprises a gas discharge plasma light source generating a limitednumber of spectral lines, a gas container devoid of surface charges anddisturbing fields containing a gas, means for injecting the light fromsaid plasma light source into said gas container to generatephotoelectrons having a plurality of energy levels from the gasmolecules contained therein, said gas container also being provided withan aperture through which said generated electrons leave the container,an impact chamber for a sample to be analyzed, said impact chamber beinglocated to position said sample to be bombarded by generated electronsleaving said aperture, energy discriminating means located between saidaperture and said impact chamber for limiting the passage therethroughof electrons having only a single kinetic energy, and means formeasuring the change in energy or diffraction of said generatedelectrons subsequent to impact with a sample.

2. Apparatus according to claim 1, characterized in that the innersurface of the gas container is provided with a layer of colloidalgraphite for eliminating the forming of field generating surfacecharges.

3. Apparatus according to claim 1, characterized in that said lightsource is a neon plasma.

4. Apparatus according to claim 1, characterized in that said gas isargon.

1. Apparatus for bombarding a sample with electrons having a limitednumber of discrete kinetic energies, characterized in that the apparatuscomprises a gas discharge plasma light source generating a limitednumber of spectral lines, a gas container devoid of surface charges anddisturbing fields containing a gas, means for injecting the light fromsaid plasma light source into said gas container to generatephotoelectrons having a plurality of energy levels from the gasmolecules contained therein, said gas container also being provided withan aperture through which said generated electrons leave the container,an impact chamber for a sample to be analyzed, said impact chamber beinglocated to position said sample to be bombarded by generated electronsleaving said aperture, energy discriminating means located between saidaperture and said impact chamber for limiting the passage therethroughof electrons having only a single kinetic energy, and means formeasuring the change in energy or diffraction of said generatedelectrons subsequent to impact with a sample.
 2. Apparatus according toclaim 1, characterized in that the inner surface of the gas container isprovided with a layer of colloidal graphite for eliminating the formingof field generating surface charges.
 3. Apparatus according to claim 1,characterized in that said light source is a neon plasma.
 4. Apparatusaccording to claim 1, characterized in that said gas is argon.